The invention relates to an apparatus for the pretreatment and subsequent conveying, plastification, or agglomeration of plastics.
The prior art reveals numerous similar apparatuses of varying design, comprising a receiver or cutter compactor for the comminution, heating, softening and treatment of a plastics material to be recycled, and also, attached thereto, a conveyor or extruder for the melting of the material thus prepared. The aim here is to obtain a final product of the highest possible quality, mostly in the form of pellets.
By way of example, EP 123 771 or EP 303 929 describe apparatuses with a receiver (receiving container) and, attached thereto, an extruder, where the plastics material introduced into the receiver is comminuted through rotation of the comminution and mixing implements and is fluidized, and is simultaneously heated by the energy introduced. A mixture with sufficiently good thermal homogeneity is thus formed. This mixture is discharged after an appropriate residence time from the receiver into the screw-based extruder, and is conveyed and, during this process, plastified or melted. The arrangement here has the screw-based extruder approximately at the level of the comminution implements. The softened plastics particles are thus actively forced or stuffed into the extruder by the mixing implements.
It is also known in principle to use twin-screw extruders and to attach these to such cutter compactors.
Many of these designs, which have been known for a long time, however, are unsatisfactory in respect of the quality of the treated plastics material obtained at the outgoing end of the screw, and/or in respect of the throughput of the screw. Particularly when using twin screws, special considerations apply which are not transferable from the results obtained with single screws.
Depending on the axial spacing between the screws and their relative direction of rotation, a distinction can be made between co-rotating and counter-rotating as well as tangential and intermeshing twin-screw conveyors or extruders.
In the case of counter-rotating screws, the two screws rotate in opposite directions.
Each of these types has particular fields of use and intended uses. In co-rotating twin-screw extruders, the conveying and the build-up of pressure are brought about in essence by the friction of the material rotating with the screw against the stationary housing wall, and the conveying results mainly from a drag flow. In counter-rotating twin-screw extruders, on the other hand, the principle of forced conveying is predominant.
Critical to the end quality of the product are, firstly, the quality of the pretreated or softened polymer material that enters the conveyor or extruder from the cutter compactor, and, additionally, the situation at intake and on conveying or, where appropriate, extrusion. Relevant factors here include the length of the individual regions or zones of the screw, and also the screw parameters, such as, for example, screw thickness, flight depths, and so on.
In the case of the present cutter compactor/conveyor combinations, accordingly, there are particular circumstances, since the material which enters the conveyor is not introduced directly, without treatment and cold, but instead has already been pretreated in the cutter compactor, viz. heated, softened and/or partly crystallized, etc. This is a co-determining factor for the intake and for the quality of the material.
The two systems—that is, the cutter compactor and the conveyor—exert an influence on one another, and the outcomes of the intake and of the further conveying, and compaction, where appropriate, are heavily dependent on the pretreatment and the consistency of the material.
One important region, accordingly, is the interface between the cutter compactor and the conveyor, in other words the region in which the homogenized pretreated material is passed from the cutter compactor into the conveyor or extruder. On the one hand, this is a purely mechanical problem area, requiring the coupling to one another of two differently operating devices. Moreover, this interface is tricky for the polymer material as well, since at this point the material is usually, close to the melting range, in a highly softened state, but is not allowed to melt. If the temperature is too low, then there are falls in the throughput and the quality; if the temperature is too high, and if unwanted melting occurs at certain places, then the intake becomes blocked.
Furthermore, precise metering and feeding of the conveyor is difficult, since the system is a closed system and there is no direct access to the intake; instead, the feeding of the material takes place from the cutter compactor, and therefore cannot be influenced directly, via a gravimetric metering device, for example.
It is therefore critical to design this transition not only in a mechanically considered way, in other words with an understanding of the polymer properties, but at the same time to consider the economics of the overall operation—in other words, high throughput and appropriate quality. The preconditions to be observed here are in some cases mutually contradictory.
Counter-rotating multiscrew or twin-screw conveyors generally have a very good intake behaviour. However, the back-ventilation at the charging aperture is poor. The amount of shear and the rate of shear or the introduction of energy into the material of a counter-rotating twin screw is nevertheless low. The distributive mixing effect of such systems is also known from experience to be worse than in the case of a single screw and co-rotating twin screws. With such systems, however, it is possible to build up a suitable pressure for connecting appropriate implements, such as e.g. profile nozzles, at the extruder end.
Specifically in systems in which a conveyor or extruder is connected to a cutter compactor, the intake or feeding into the twin-screw conveyor is far from easily adjustable and e.g. the metering cannot take place via a gravimetric metering device. In contrast, in the cutter compactor, the circulating mixing and comminution implements give rise to a continuous feeding of the pretreated, softened particles or a continuous material flow towards the intake aperture of the conveyor or extruder.
Added to this is the fact that a further common feature of the known devices is that the direction of conveying or of rotation of the mixing and comminution implements, and therefore the direction in which the particles of material circulate in the receiver, and the direction of conveying of the extruder, in particular of an extruder, are in essence identical or have the same sense. This arrangement, selected intentionally, was the result of the desire to maximize stuffing of the material into the screw, or to force-feed the screw. This concept of stuffing the particles into the conveying screw or extruder screw in the direction of conveying of the screw was also very obvious and was in line with the familiar thinking of the person skilled in the art, since it means that the particles do not have to reverse their direction of movement and there is therefore no need to exert any additional force for the change of direction. An objective here, and in further derivative developments, was always to maximize screw fill and to amplify this stuffing effect. By way of example, attempts have also been made to extend the intake region of the extruder in the manner of a cone or to curve the comminution implements in the shape of a sickle, so that these can act like a trowel in feeding the softened material into the screw. Displacement of the extruder, on the inflow side, from a radial position to a tangential position in relation to the container further amplified the stuffing effect, and increased the force with which the plastics material from the circulating implement was conveyed or forced into the extruder.
Apparatuses of this type are in principle capable of functioning, and they operate satisfactorily, although with recurring problems:
By way of example, an effect repeatedly observed with materials with low energy content, e.g. PET fibres or PET foils, or with materials which at a low temperature become sticky or soft, e.g. polylactic acid (PLA) is that when, intentionally, stuffing of the plastics material into the intake region of the extruder or conveyor, under pressure, is achieved by components moving in the same sense, this leads to premature melting of the material immediately after, or else in, the intake region of the extruder or of the screw. This firstly reduces the conveying effect of the extruder, and secondly there can also be some reverse flow of the said melt into the region of the cutter compactor or receiver, with the result that flakes that have not yet melted adhere to the melt, and in turn the melt thus cools and to some extent solidifies, with resultant formation of a clump or conglomerate made of to some extent solidified melt and of solid plastics particles. This causes blockage on the intake of the extruder and caking of the mixing and comminution implements. A further consequence is reduction of the throughput or quantitative output of the conveyor or extruder, since adequate filling of the screw is no longer achieved.
Another possibility here is that movement of the mixing and comminution implements is prevented. In such cases, the system normally has to be shut down and thoroughly cleaned.
Problems also occur with polymer materials which have already been heated in the cutter compactor up to the vicinity of their melting range. If overfilling of the intake region occurs here, the material melts and intake is impaired.
Problems are also encountered with fibrous materials that are mostly orientated and linear, with a certain amount of longitudinal elongation and low thickness or stiffness, for example plastics foils cut into strips. A main reason for this is that the elongate material is retained at the outflow end of the intake aperture of the screw, where one end of the strip protrudes into the receiver and the other end protrudes into the intake region. Since the mixing implements and the screw are moving in the same sense or exert the same conveying-direction component and pressure component on the material, both ends of the strip are subjected to tension and pressure in the same direction, and release of the strip becomes impossible. This in turn leads to accumulation of the material in the said region, to a narrowing of the cross section of the intake aperture, and to poorer intake performance and, as a further consequence, to reduced throughput. The increased feed pressure in this region can moreover cause melting, and this in turn causes the problems mentioned in the introduction.
For counter-rotating twin-screw conveyors too, therefore, the intake is sensitive.